The present invention relates to a motor, or more in particular to a piezoelectric motor using an electro-mechanical transducer such as a piezoelectric material.
A conventional ultrasonic motor using a piezoelectric material is described in "Ceramics", 21 (1986), No. 1, pp. 9 to 14, "Applied Physics", 54 (1985), No. 6, pp. 589 to 590, and others.
The ultrasonic motor that was realized for the first time is of a vibrating-reed type. This type of the ultrasonic motor is for converting a longitudinal vibration of a piezoelectric member into an elliptical motion of the forward end of a vibrating reed which prods a moving element (slider) into motion. ("Ceramics", p. 10, FIG. 1)
The ultrasonic motor of this type has a thin vibrating reed with a small area of the forward end thereof, and therefore cannot produce a large torque on the one hand and poses the problem of short service life for low wear resistance on the other.
In order to solve this problem of endurance, an ultrasonic motor of travelling-wave type was conceived ("Ceramics", p. 10, FIG. 3). This type of ultrasonic motor utilizes the fact that when a travelling wave is caused in an elastic material, particles in the surface thereof are set in an elliptical motion, and is similar to the ultrasonic motor of vibrating-read type in that both are driven by friction due to elliptical motion.
On the other hand, a travelling wave is produced by superposing, for example, two standing waves different in time and spatial phase by .pi./2 respectively. In the case of a linear motor, in order to eliminate the effect of a wave reflected from the boundary of an elastic member, the end of the elastic member is curved ("Applied Physics", Vol. 54, No. 6 (1985), p. 589, FIG. 1) to cause the travelling wave to make a round in the surface, or a vibration absorber is mounted at the end thereof (See the same issue of the magazine at p. 590, FIG. 4). A rotary motor is constructed with two standing waves superposed on a ring (See the same issue of the magazine, at p. 590, FIG. 7).
The ultrasonic motor of travelling-wave type, as compared with the motor of vibrating-reed type, has an increased contact area and therefore has an improved wear resistance.
In the above-mentioned ultrasonic motor of travelling-wave type, though more advanced than the vibrating-reed type, the elastic member with a travelling wave excited therein is still in linear contact with a slider providing a moving unit. The disadvantage of a small contact area is not only a cause of a small wear resistance but results in a reduced driving force and a lower output efficiency due to the elastic deformation of the slider or the like.
In order to solve this problem, JP-A-61-102177 and JP-A-61-203872 disclose a device comprising a slider 35 and elastic material member 36 which is deformed to increase the contact area for an increased driving force (FIG. 13). Nevertheless, the energy loss caused by the elastic deformation of the slider 36 reduces the efficiency. Further, the velocity component 38 of surface particles of the elastic member along the normal to the sliding surface, that is, that of the transverse wave increases while the component 37 of the longitudinal wave that provides a thrust extremely decreases with the distance from the wave front. The thrust is thus not increased considerably.
In driving an ultrasonic motor of travelling-wave type, it is necessary that the amplitude of the transverse wave be sufficiently large as compared with the surface roughness of the slider and the elastic member. For this purpose, a voltage of more than several volts is applied to the piezoelectric member of an ultrasonic motor of rotary type and several hundred volts to that of linear type to excite the travelling wave. The ratio of amplitude between the longitudinal and transverse waves, on the other hand, is fixed for each elastic member. As a result, the distance covered by each period of excitation is about a micron, which is substantially the degree of positional accuracy attained when the motor is used as an inching mechanism.
If the applied voltage to the piezoelectric member is increased to increase the velocity, the amplitude of the transverse wave that does not contribute to the moving speed of the slider is increased for an increased energy loss.
Generally, the excitation frequency of an ultrasonic motor is several ten KHz with the wavelength of several cm. This wavelength is the greatest bottleneck in reducing the size of the ultrasonic motor. Especially in the case of the linear motor, a construction with a curved end of the elastic member mentioned above to dampen the effect of the reflected wave requires a sufficiently large radius of curvature as compared with the wavelength, thus making it impossible to further reduce the size.
In a linear motor with transducers mounted at the ends of a metal bar, the excitation is not derived from the resonant frequency of the metal bar, and therefore a large, strong excitation mechanism is required ("Ceramics", 21, (1986), No. 1). If the travelling wave is to be absorbed and to realize a reciprocal motion, excitation mechanisms are required at both the ends. It is thus difficult to reduce the size. Further, a large and strong excitation mechanism causes a greater energy loss.
The magnitude of the wavelength is not only a bottleneck to a reduced size but leads to the disadvantage that a large contact area between the slider and the transducer is not secured. The linear motor described above uses an inflected wave to shorten the wavelength, which wave has a wavelength of, say, 43 mm with an aluminum bar of 6 mm square excited at 27 KHz. The wavelength is given as .lambda.=2.pi.(EI/.rho.A)1/4.omega..sup.-1/2 (E: Young's modulus, A: Sectional area, .rho.: Density, and I: Second moment of area). Therefore, the wavelength is not reduced by one half for example, unless the sectional area of a transducer is increased 16 times. It is thus not an easy matter to increase the number of wave fronts in contact with the slider.
The transducer or the elastic member described above is made of duralumin or the like of high Young's modulus in order to improve the propagation efficiency of the elastic wave. This greatly limits the material making up the area in contact with the elastic member in view of the output efficiency.